The present invention relates to a measurement and adjustment device and method applied to a wire-based tool-electrode which is inclined relative to a main direction in an electrical discharge machine. Such a machining method commonly called taper machining is widely used in the field of wire electrical discharge machines. These machines are usually fitted with a plane on which the piece to be machined is attached or work table plane; with a bottom wire-guide movable in a plane XY parallel to the work table plane; with a top wire-guide movable in a plane UV also parallel to the work table plane. The top guide may moreover be moved along a fifth axis Z perpendicular to the work table plane. The electrode is stretched between the two wire-guides with a sufficient mechanical load for it to approach a rectilinear shape in the active portion. The two wire-guides move under the guidance of a numerical control programmed to construct ruled surfaces from the most basic to the most complex, which the machine can produce with precisions of the order of a few microns. The active portion of the wire electrode is similar to a rectilinear cylinder with a diameter of approximately 0.01 to 0.3 mm and a length that may extend from a few mm to 50 cm as an example.
In document U.S. Pat. No. 4,559,601, there is a description of the typical issues to be addressed by taper machining processes. In case of taper angle variations during machining, the trajectory followed by the bottom guide in the plane XY is different from the trajectory followed by the top guide in the plane UV; the one is not deduced simply from the other. In summary, the correlated management of travel on one or other of these two trajectories requires an exact knowledge of the position Z of each of the guides relative to the work table plane. In this document, the assumption is made that the axis of the wire is similar to a rectilinear segment stretched between two pinpoint guides. Such a simplification was rapidly seen to be inadequate in matters of precision machining. As will be explained hereinafter, account must be taken of the mechanical stresses which deform the wire in the vicinity of the guides so as to delimit the rectilinear active portion thereof that is in practice usable for machining.
Document U.S. Pat. No. 4,736,086 helps to understand how the trajectories imposed on each of the two guides by the numerical control must be corrected so that the final geometry of the machined piece is correct. For this purpose, offsets which take account notably of the machining gap, the radius of the wire, its inclination, the movement of the resting (contact) point of the wire caused by the rounded shape of the guides at the output, etc. are applied to the desired final shape on the piece. In particular, the movement of the resting (contact) point of the wire on the guide as a function of its inclination is computed therein on the assumption that the guides trap the wire with a clearance close to zero, are of perfect axial geometry and comprise a well known output radius—all things that are not easy to obtain in practice due to the difficulties and high costs of ensuring tight tolerances during the production of these small-dimensioned guide members in sapphire or diamond.
On the other hand, the same document describes why, due to its rigidity, the wire does not perfectly conform to the roundness of the guide and teaches how to compute an additional correction with the aid of a model of the bending of the wire in the elastic domain in which in particular the mechanical load applied to the wire, its moment of inertia, its modulus of elasticity, etc. come into play. The weakness of such a model is that it assumes the mechanical load to be constant despite the variations in inclination and in the unwinding speed of the wire. Furthermore, the clearance in the guides has to be known in advance and must remain constant despite wear.
The invention does still require the provision of measurement cycles in order to calibrate and recalibrate certain parameters of the model. In addition, experience has shown that it is easy to obtain a sufficiently regular axis-symmetric shape of the wire-guides. On the other hand, obtaining a constant wire guide output radius is much more difficult. Therefore, it is impossible to predict the actual height of the pivot point with the aid of a model when the angle of inclination of the wire varies.
Document CH 690 420 deals with the use of closed guides having an axial symmetry and used to machine with large taper angles of the wire. The document describes the stresses inflicted on the wire when it leaves the top guide and abruptly changes direction. These stresses may cause the wire to enter the domain of plastic deformations. The invention teaches which minimal radius to give to the guide so that the wire does not transport any plastic deformations in its active portion. When the wire unwinds from top to bottom, the plastic deformations caused by the bottom guide do not have to be taken into account. The precautions recommended in the document are used to ensure (see FIG. 1) that the wire 1 is similar to a rectilinear cylindrical segment 6 of small diameter stretched between two pivot points W1, W2, one being close to the bottom guide 2 and the other to the top guide 4. The problem is then limited to identifying the heights Zw1, Zw2 of the said pivot points, thus making it possible with the aid of well known computation methods, to determine the corrections applicable to the trajectories of the guides 2 and 4.
More particularly, document CH 690 420 teaches how to use automatic measurement cycles to determine the heights Zw1, Zw2 with the aid of an eyepiece 8 (see FIG. 2). The wire searches for the center of said eyepiece at two heights of the top wire-guide Zmin and Zmax. At the height Zmin the wire is inclined in a first direction by an angle β, then at the height Zmax it is inclined in a second opposite direction by an angle α which would be required to be equal to β so that the proposed formula gives an exact result. Unfortunately with this method, to ensure that α=β, it would be necessary to know the exact heights of the guides and those of the pivot points; in FIG. 2, the latter are represented as coinciding with the guides. Because of this inadequate knowledge, the procedure therefore begins with approximate data which is corrected progressively during several iterations of the computation. The wasted time is acceptable if one or two inclinations of the wire are to be calibrated; however, if it is necessary to prepare a machining process comprising many taper angle values, the complete cycle of the iterations must be repeated for each wire inclination value, hence a considerable waste of time.
There are other disadvantages to add to the lack of effectiveness of the method for multi-angle calibrations:                the base plane of the measurement eyepiece must first be set parallel to the work table plane;        the wire must first be set perpendicular to the work table plane;        the center of the eyepiece must, at each step, be again identified by a series of approaches in crossed directions;        it is necessary to determine whether the approaches take place on the top edge of the eyepiece or on its bottom edge;        at the beginning, the inadequate knowledge of the exact heights of the guides and of the pivot points brings risks of collisions with the eyepiece, related to the vertical movements of the Z axis.        
All these factors imply that the measurement method is not very reliable, unnecessarily complex and costly in execution time.
Finally, for the method in question to be acceptable, it has to be assumed that the guides have a perfect axis-symmetric shape and that their axis of symmetry is parallel to the direction Z. At a certain level of precision, such a hypothesis must be abandoned for at least two reasons. Firstly, nothing guarantees (see FIG. 3) that the axes of symmetry 3 and 5 of the guides are parallel to Z. The axis Z′ of FIG. 3 represents the position of the wire ideally set perpendicular to the work table plane or parallel to the axis Z. Then the two machining contacts 15 and 16 transmit through the guides a defect that tends to deviate the wire out of alignment in its active portion, thus altering the position of the pivot points W1, W2. This shows the value of identifying the pivot points not only as a function of the inclination ΔUV of the wire but also, for each value ΔUV, as a function of the direction in the plane UV of the said inclination; the term {right arrow over (ΔUV)} will be used hereinafter to designate an inclination of the wire oriented in a plane UV.
FIG. 4 illustrates a similar method routinely used, which is mentioned for example in documents U.S. Pat. No. 5,003,147 or U.S. Pat. No. 5,006,691 and which uses a mechanical gauge device comprising two reference abutments 7, 8 placed one above the other, exactly aligned in a plane perpendicular to the work table plane and of which the exact height difference H is known. By inclining the wire in one direction and then the other relative to its vertical position, one or the other of these reference abutments is contacted.
The operation usually proceeds in 7 steps marked from <1a> to <7a> in FIG. 4.
Step <1a>: the wire is set perfectly perpendicular to the work table plane.
Step <2a>: the wire is brought into contact with the two abutments 7 and 8. The position xy1 reached is measured and stored.
Step <3a>: return to approximately the position <1a> to be able to incline the wire without risk of collision.
Step <4a>: the wire is inclined towards the left by making a movement −ΔUV.
Step <5a>: the wire is brought into contact with the abutment 8. The position xy3 reached is measured and stored.
Step <6a>: the wire is inclined towards the right by making a movement +ΔUV from its vertical position.
Step <7a>: the wire is brought into contact with the abutment 7. The position xy2 reached is measured and stored.
If the wire is vertical at the beginning of the operation, then the triangles JKL and IKM are similar and it is possible to compute the distances D1 and D2 which will be used to ascertain the height of each of the two pivot points relative to the work table plane. These results D1 and D2 obtained by three approaches at <2a>, <5a> and <7a> depend a priori only on the accuracy of the approaches against the two reference abutments and on the accuracy of the dimension H; these accuracies are routinely of the order of a micron. However, when the wire is close to the vertical, it is not possible to determine with certainty whether the two reference abutments contact the wire at the same time or only one or the other. Accordingly, it is necessary to install the measurement device truly perpendicular to the work table plane, to set the wire parallel to the two reference abutments as well as perpendicular to the work table plane. These operations are still lengthy and tricky. It is therefore again necessary to take account of the uncertainties relating to the latter to assess the accuracy of the results and correct them. Finally, as in the preceding device and method, the wire has to be inclined in two opposite directions to compute the heights of the pivot points tied to an inclination value ΔUV. This is acceptable provided only that each of the two guides comprises a perfect axial symmetry as already mentioned hereinabove.